One key task to identify and plan for migration to Lync Server 2013 is to find the right timing to rollout the latest version of Lync 2013 client. This typically takes place at the final stages prior to decommission of the existing version of Lync/OCS Server backend.
The good news is that, unlike in the previous Lync Server 2010 version, existing Lync 2010 (N-1) as well as Office Communicator 2007 R2 (N-2) level clients are supported when a UC-enabled user account is homed on a Lync Server 2013 Front-End Pool. Clearly, new features such as integrated Persistent Chat, multi-party video, etc. will not be available while running older versions of supported Lync/MOC clients (N-1, N-2).
Even better, the converse is true; that is to say, you can safely deploy the Lync 2013 client first and be rest assured that it will work with a backend still running Lync Server 2010* while you continue to put the pieces of the 2013 infrastructure in place in a side-by-side co-existence scenario.
MOC 2007 R2 and Lync 2010 Group Chat clients, now known as Persistent Chat, behave a bit differently. Lync 2013 client (with integrated Persistent Chat feature) does not function with a N-2 level Group Chat Server (OCS 2007 R2). N-2, N-1 and N level Group/Persistent Chat clients all work with Lync Server 2013 Persistent Chat server backend.
* not OCS 2007 R2 though